This invention relates to photolithographic fabrication of planar structures and, more particularly, to contact printing techniques using liquid photoresists.
Planar structures for microsized circuitry and small semiconductor devices are often fabricated by photolithographic masking techniques. As applications for high frequency circuits increase, greater precision of these miniature circuits and devices is required, and accordingly, the photolithographic techniques must provide increased accuracy and reproducibility.
The basic form of photolithographic masking utilizes a process called contact printing. In this process, a layer of light-sensitive material is applied to a wafer, and a mask, having opaque and transparent portions arranged in a preformed pattern thereon, is placed in contact with the applied layer. Light is then passed through the preformed mask so that an image of the desired pattern is formed on the light-sensitive material. The solubility of this material, termed a photoresist, changes when exposed to optical frequency radiation, especially in the ultraviolet range; negative photoresists become less soluble by illumination and positive photoresists become more soluble when illuminated. However, with illumination through an appropriate mask pattern, either type of photoresist can be used to form and then, with its corresponding solvent, to develop a desired pattern in the photoresist. Selective etching of the underlying wafer material follows in accordance with the developed photoresist pattern. Then the remaining photoresist is removed, leaving a duplicate of the mask pattern in the wafer.
It is essential for high-quality photolithographic reproduction by contact printing for the mask to be maintained in good physical contact with the photoresist during the illumination. If it is not, the light image on the photoresist will be blurred and will not replicate the pattern formed in the mask. Any bumps or irregularities in the bearing surface of the mask, or in the surface of the photoresist, will preclude the requisite contact. Conventionally, the photoresist material is applied in liquid form to a rotating wafer. The spinning technique is used to insure a flat surface of the photoresist. However, centrifugal forces inherently produce a build-up of the photoresist material at the periphery of the wafer and this ridge obviously prevents the mask from being placed in intimate contact with the photoresist.
In addition, other irregularities in the photoresist surface, or the bearing surface, result in loss of physical contact and degraded reproduction.